Eligible patients with documented cancer or non-cancer pain, either currently receiving a WHO step II opioid and requiring the initiation of a WHO step III opioid or currently receiving a WHO step III opioid, and having opioid-related constipation defined by either a KESS score ≥9 or the current use of laxatives (≥3 times per week), will be randomly assigned to receive either OXN PR or Oxy PR. Randomisation will be stratified on the cause of pain: cancer or non-cancer.